1. Field of the Invention
This invention relates to measurement techniques and, in particular, to measurement techniques involving polymer bodies.
2. Art Background
The use of particulate matter blended with polymers such as polyvinyl chloride (PVC) is quite common. Many widely used commercial products such as wire coatings and wire jacketings rely on such blends to produce properties necessary for their appropriate functioning. For example, it has been found that polymers such as PVC degrade when exposed to ultraviolet light. Various additives, e.g., carbon black, have been incorporated into PVC to avoid degradation during applications such as outdoor wiring where ultraviolet exposure is relatively high.
Additionally, particulate additives other than carbon black are employed to modify the properties of polymers. For example, particulate titanium oxide is often added to polymers such as polyethylene and PVC. A uniform distribution of these particulate additives at an appropriate concentration level in the polymer body significantly influences additive efficacy. (If the concentration is not uniform, typically the additive is less efficacious.) Thus, a quality control procedure historically has been considered necessary to ensure that appropriate particulate additive levels are employed. Initially in the case of carbon black, a simple microscopic examination was utilized. In this technique, a sample was viewed under an optical microscope and subjectively compared to a sample which had the desired properties. If the comparison was satisfactory, the sample was also considered satisfactory. Obviously, this method is very qualitative and often lead to the use of material which ultimately proved to be unsatisfactory.
In an attempt to quantify concentration measurements a spectroscopic technique was then developed. This procedure (which was eventually standardized as ASTM D3349, described in the 1981 Annual Book of The ASTM Book of Standards, American Society for Testing Materials, Part 36, 829-834) involved the preparation of a very thin sample whose absorption is measured. The quantity of light absorbed by the sample is correlated with a concentration of the carbon black in the initial sample.
Although the absorption test is significantly more quantitative than optical microscopy, it is also substantially more complicated and time consuming. The absorption test requires the preparation of a very thin compression molded film which must be completely pinhole free. (Films thicker than approximately 0.0005 inch substantially attenuate incident light. To overcome this problem, expedients must be employed which significantly reduce the accuracy of the procedure.) However, the preparation of such a thin film requires extraordinary skill. Even once the sample is prepared, its transfer to the absorption spectrophotometer is quite difficult. The film tends, through electrostatic interactions, to cling to the material upon which it is prepared. Thus, transfer often results in either torn, useless films or films which have been stretched to produce nonuniformities. Due to induced nonuniformity and the use of undesirably thick films, substantial disagreement between measurements made at different facilities on the same sample is often produced.
The absorption technique is more quantitative than the initial microscopic procedure, but it is not entirely consistent and the improvement over microscopy is balanced by a significant increase in complexity and preparation time. However, both increased complexity and time consumption are undesirable for applications such as those involved in quality control.